1. Field of the Invention
The present invention relates to a high-frequency oscillator for use in a millimeter-wave-band or microwave-band radio communication apparatus, radar device and the like.
2. Description of the Related Art
In the case where millimeter wave band or microwave band is used for communication or radar applications, it is unlikely to be subject to influences of weather or powder dust. Because it is of short wavelength, a transmitting/receiving antenna can be made compact. In addition, it enables to detect the relative speed to an object through the Doppler effect with high precision. For these reasons, in recent years, millimeter-wave radar has been started to be installed in vehicles to prevent vehicle collisions due to carelessness or misjudgment of drivers.
A conventional millimeter-wave oscillator includes a semiconductor device such as a Gunn diode. In such a millimeter-wave oscillator including a semiconductor device, the temperature of the semiconductor device becomes high when driven, and the oscillation characteristics change. For this reason, it is necessary to devise a structure so as to suppress the temperature increase at driving. However, such a millimeter-wave oscillator must have a complicated structure (see, for example, JP-A-2005-19570), which has been a problem.
On the other hand, in a communication device for communicating information via radio or wire, a high-frequency filter having a function to take out only a desired frequency band is built in as its major component. In order to use frequency effectively to operate a communication device with saved energy, the high-frequency filter is required to be excellent in attenuation characteristics and to have a small insertion loss. To realize the requirement, a resonant element of a high Q value is needed. As a high-frequency filter showing a high Q value, there is known a filter using a superconductive element (see, for example, JP-A-2004-349966). However, in order to operate a superconductive element, the element must be cooled by use of liquid nitrogen or liquid helium. As a consequence, it is difficult to realize a compact high-frequency filter, and it is also difficult to apply a superconductive element to consumer products from the viewpoint of cost.
In contrast to these conventional oscillators, an oscillator employing a quite different physical principle has been proposed. See, for example, S. I. Kiselev et al., Nature, 425, 308 (2003). This oscillator has a stacked film including a magnetic layer, a nonmagnetic metal layer and a magnetic layer, and a pair of electrodes for allowing current to flow perpendicularly to the plane of the stacked film. An oscillation phenomenon is generated when a current is supplied. Hereinafter, this oscillator is referred to as a current-perpendicular-to-plane (CPP) oscillator. With regard to the CPP oscillator, not only applications as an oscillator, but also other applications to a high-frequency filter element and the like are now under discussions (see U.S. Pat. No. 5,695,864).
In this CPP oscillator, the element size of 100 nm×100 nm or less is required from its physical principle. Such an ultrafine oscillator using not a semiconductor but a metal is expected to have advantages including excellent heat dissipation, small heat generation, and small temperature characteristics of oscillation characteristics. Further, if such a circuit is used, it is not necessary to construct a complicated structure as in the conventional technique for attaining an oscillation phenomenon. The CPP oscillator has a unique characteristic in principle of operation that an oscillation phenomenon is attained only by flowing direct current to the device. Accordingly, it has an expected advantage that in forming a high-frequency oscillator, the circuit thereof can be simplified.
However, in a normal CPP oscillator, it is difficult to obtain operations as a practical device, and at present, there is no development on devices for practical use. This is because an output of oscillation signals is extremely low, and when the amount of current is increased to make high the output of oscillation signals, magnetization reversal of the magnetic layer occurs due to a spin transfer torque phenomenon. See, for example, J. C. Slonczewski, J. Magn. Magn. Mater., 159, L1 (1996). Once the magnetization reversal occurs, oscillation conditions change, and stable oscillation operations cannot be maintained, and as a result, it cannot be used as a device. In addition, it is not easy to change oscillation frequencies in a normal CPP oscillator, and its operating frequency waveband is limited, which has been another problem.